1. Field of the Invention
The present invention relates to unique hot-melt adhesive compositions, and, more especially, to unique hot-melt adhesive compositions which, after application, crosslink irreversibly in a few days under the effect or influence of atmospheric moisture/humidity.
2. Description of the Prior Art
It is known to this art that a hot-melt adhesive is a formulation which is solid at ordinary temperature and applied in the molten state (at approximately 180.degree. C.), and which hardens on cooling. Such hot-melt adhesives typically contain two principal constituents: a thermoplastic polymer (responsible for good mechanical properties) and a tackifying resin (serving a function where adhesion, tackiness when hot, fluidity and wettability are concerned). A number of additives such as waxes, stabilizers, fillers, etc., are usually included. The thermoplastic polymers most widely employed are ethylene/vinyl acetate copolymers (hereinafter "EVA"), atactic poly-.alpha.-olefin ("APAO"), thermoplastic rubber, and others. The tackifying resins are typically selected from among three broad classes: rosins (and derivatives thereof), terpene resins and petroleum-derived resins (aliphatic, aromatic and others). From an industrial standpoint, these hot-melt adhesives present a great number of advantages, which account for their increasing usage:
(a) easy application in automated facilities,
(b) short setting time (a few seconds), permitting high adhesive bonding speeds,
(c) very good adhesion to a wide variety of substrates: paper, wood, cardboard, textiles, plastics and others,
(d) reasonable cost.
However, because of their thermoplasticity, hot-melt adhesives offer only limited heat resistance under load, limited in most instances to 70.degree.-80.degree. C. (100.degree. C. maximum), which precludes their use for certain applications, such as: motor vehicle, building construction, textile, veneering and high-quality bookbinding.
Considerable efforts have been undertaken to remedy this disadvantage.
For a number of years, a wide variety of compositions have been formulated in efforts to avoid the above drawback. These can be classified into two-component products and single-component products. By "two-component" is intended a formulation which is necessarily presented in the form of two separate products which must be mixed at the time of use, because the mixture itself is reactive and because the properties of the final product change as soon as it is produced. By "single-component" product is intended a single unitary product which is stable in storage, as least for a reasonable period of time. The two-component product approach, especially epoxide-based, is cumbersome and tricky. That of the one-component products is more developed, and it is along this line that most of the improvements have ensued.
Thermoplastic polymers of high melting point have been considered in this respect. Commercial products (based on polyamides or polyesters) have a good heat resistance. However, other disadvantages are presented:
(i) difficulty in processing (too high viscosity),
(ii) thermal degradation prior to deposition, because of the very high processing temperatures which are required,
(iii) short assembly times (time available for effecting the adhesive bonding, from the instant when the molten adhesive is deposited onto the first support to the instant when the adhesive becomes too viscous because of cooling to properly bond to the second support),
(iv) mediocre adhesion to certain substrates ("glazing").
Reactive formulations have also been considered which crosslink after bonding. These can be classified into a number of variants, described below.
Thermal crosslinking is attained, for example, by including a blocked isocyanate in a formulation containing hydroxyl groups. To effect the crosslinking, the adhesive must be heated above its application temperature. During this stage, the adhesive is again converted through the liquid state; it is therefore absolutely required that the components to be bonded are maintained in place. This curing lasts in most cases for a few minutes to a few tens of minutes (cf. European Patents Nos. 0,294,271 or 0,302,620). This comprehends that one of the principal advantages of hot-melt adhesives, i.e., virtually instantaneous bonding permitting high rates, is lost, and the use of these particular products is reserved for special cases.
Formulations which chemically crosslink by atmospheric oxygen include alkylboranes. These are more experimental than industrial in nature.
Crosslinking via UV irradiation has been proposed by Dynamit Nobel using polyesters which are sensitive to UV radiation, which permits processing at about 50.degree. C. and, according to the same principle, by Shell, in respect of its product Kraton 1320 X. This route is applicable only to the crosslinking of products having slight thicknesses, and its application is highly specific (for example, application to coating products).
As regards crosslinking at ordinary temperature, no manipulation of the joint is necessary after bonding. The processing is therefore as simple as that of a conventional hot-melt. The crosslinking often occurs under the effect of atmospheric moisture, by virtue of excess isocyanate functional groups in the formulation. Commercial products of this type (described, for example, in European Patent No. 0,107,097--Fuller) which are employed in laminating, or used in motor vehicles, comprise polyurethane oligomers having isocyanate endgroups. Because of the limited stability of isocyanate functional groups, these formulations are used at a lower temperature (typically 130.degree. C.). Heat resistance and chemical resistance after crosslinking are very good. On the other hand, the rapidity of setting (on the order of one minute) and the initial creep resistance (during the first 24 hours), which are greatly inferior to those of the known hot-melt adhesives, with the requirement for storage in the absence of moisture, circumscribe the weak links in this solution. Alternatives are described, for example, in European Patent No. 0,293,602 (Fuller).
And EP-A-0,380,379 describes a crosslinkable hot-melt adhesive composition based on EVA copolymer containing hydroxyl and polyisocyanate functional groups, in the form of a prepolymer containing free isocyanate functional groups, prepared by reacting the EVA copolymer containing hydroxyl functional groups with excess polyisocyanate. Such hydroxylated EVA copolymer contains from 20 to 150 meq. OH/100 g of polymer, which react with the excess isocyanate functional groups during polymerization, resulting in the curing by reaction of the NCO groups with environmental water.